In order to afford maximum protection to vehicular traffic as well as to pedestrians at railroad-highway grade crossings, it is normal practice to provide a highway crossing gate and signal assembly which includes warning signs and a bell, flashing lights and a gate operating mechanism which moves an elongated gate arm between a vertical clear position and a horizontal traffic blocking position. However, many existing crossings are only provided with warning signs, or with warning signs, bell and flashing lights, or with warning signs, bell, flashing lights and operating mechanisms for actuating wooden gate arms for controlling oncoming traffic. Presently, there is a general trend to replace any of the aged and/or damaged wooden gate arms with a new version of lightweight gate arms that may be more readily and easily installed and that are not completely destroyed when a motor vehicle strikes the gate arm. It is also a common practice to install lightweight gate operating mechanisms on existing crossings which only had warning signs, or warning signs and bell and flashing lights. However, various problems have been encountered in attempting to replace wood gate arms with lightweight gate arms and to install lightweight gate operating mechanism on existing crossings. The existing railroad-highway crossing apparatus has never been standardized so that the dimensions and distances between the crossbuck signs and the flashing lights as well as between the flashing lights themselves and the supporting masts or poles greatly vary from one crossing installation to any other. Thus, the clearances between a vertically positioned gate arm and the superstructure at each highway crossing installation had to be thoroughly examined and considered in order to insure that a gate arm would not strike any of the overhead warning signal devices. In the past, special adapters and bracket assemblies had to be individually designed and used for each distinct and different crossing installation due to the great number of variations which could occur with previously installed equipment. Further, it was virtually impossible to predict whether an operating mechanism could even be mounted at a convenient height to facilitate maintenance. It will be appreciated that the particular height that the operating mechanism is mounted from the level of the ground is determined by the required offset dimension which is the distance measured from the center of the upstanding mast and the center of the gate arm when it is in its vertical clear position. It is well known that the smaller the offset, the lower to the ground level the operating mechanism may be mounted; however, there are instances when the vertical gate arm cannot clear overhead obstacles without increasing the offset more than would ordinarily or conveniently be used. It has been found that a great many of the previous problems could be resolved and any increase in the offset could be minimized if the gate arm could be selectively moved and laterally positioned to avoid any overhead obstacle or superstructure.